Ferroelectric recording medium and writing method for the same

ABSTRACT

A ferroelectric recording medium and a writing method for the same are provided. The ferroelectric recording medium includes a ferroelectric layer which reverses its polarization when receiving a predetermined coercive voltage. A nonvolatile anisotrophic conduction layer is formed on the ferroelectric layer. A resistance of the anisotrophic conduction layer decreases when receiving a first voltage lower than the coercive voltage, and the resistance of the anisotrophic conduction layer increases when receiving a second voltage higher than the coercive voltage. Multi-bit information is stored by a combination of polarization states of the ferroelectric layer and the resistance of the anisotrophic conduction layer. Accordingly, multiple bits can be expressed on one domain of the ferroelectric recording medium.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application claims priority from Korean Patent Application No.10-2005-0011410, filed on Feb. 7, 2005, in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein in itsentirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Apparatuses and methods consistent with the present invention relate toa ferroelectric recording medium, and more particularly, to a multi-bitferroelectric recording medium and a writing method for the same.

2. Description of the Related Art

Ferroelectric material has spontaneous polarization that can be reversedby an electric field. A ferroelectric recording medium is ahigh-capacity nonvolatile recording medium on which data can be written,modified, and stored using physical characteristics of the ferroelectricmaterial.

A related art ferroelectric recording medium will be described withreference to FIGS. 1 and 2.

FIG. 1 is a perspective view of a related art ferroelectric recordingmedium, and FIG. 2 is a perspective view of another related artferroelectric recording medium.

Referring to FIGS. 1 and 2, related art ferroelectric recording mediumsinclude a ferroelectric recording layer 2 (12). Using a read/write head1 (11), data are read from or written to the ferroelectric recordinglayer 2 (22). The ferroelectric recording mediums may further include aseparate protective film 13 (FIG. 2) so as to protect the read/writehead 1 (11) and the ferroelectric recording layer 2 (22) from beingdamaged by a direct contact therebetween.

The ferroelectric recording layer 2 (22) is partitioned into a pluralityof domains 3 (14). Data is recorded in each of the domains 3 (14) andread therefrom whenever necessary. A reading/writing method for arelated art ferroelectric recording medium will now be described withreference to FIG. 3.

Referring to FIG. 3, a related art ferroelectric recording mediumincludes a ferroelectric recording layer 22 partitioned into a pluralityof domains 24, 25, 26 and 27, and a protective film 23 for protectingthe ferroelectric recording layer 22.

When a predetermined voltage is applied to a specific region of theferroelectric recording medium through a read/write head, a polarizationreversal occurs in a corresponding domain and then the domain isswitched back to the original polarization by another voltage.

Thus, an upper polarization and a lower polarization occur in eachdomain 24, 25, 26 and 27 of the ferroelectric recording layer. States ofthe domains 24, 25, 26 and 27 are defined by the upper polarization andthe lower polarization.

When the read/write head is positioned above a specific domain in whichinformation is recorded, an induced current corresponding to thepolarization of the domain is generated from the read/write head.Reference symbols V1 and V2 represent polarization amounts, andreference symbols I1 and I2 represent induced currents respectivelycorresponding to the polarization amounts (V1≠V2).

The related art ferroelectric recording medium can have only two states.That is, the upper polarization and the lower polarization in eachdomain and can store only one bit in each domain. Accordingly, there isa demand for improving the ferroelectric recording medium so that theferroelectric recording medium can have more states and more bits can berecorded in each domain.

SUMMARY OF THE INVENTION

The present invention provides a multi-bit ferroelectric recordingmedium and a writing method for the same.

According to an aspect of the present invention, there is provided aferroelectric recording medium including: a ferroelectric layer whichreverses its polarization when receiving a predetermined coercivevoltage; and a nonvolatile anisotrophic conduction layer formed on theferroelectric layer, wherein a resistance of the anisotrophic conductionlayer decreases when receiving at a first voltage lower than thecoercive voltage, and the resistance of the anisotrophic conductionlayer increases when receiving a second voltage higher than the coercivevoltage, wherein multi-bit information is stored by a combination ofpolarization states of the ferroelectric layer and the resistance of theanisotrophic conduction layer.

The multi-bit information may be stored by a combination of upward anddownward polarizations of the ferroelectric layer and high and lowresistances of the anisotrophic conduction layer.

A first state may be expressed by the downward polarization of theferroelectric layer and the low resistance of the anisotrophicconduction layer, and a second state may be expressed by the downwardpolarization of the ferroelectric layer and the high resistance of theanisotrophic conduction layer. A third state may be expressed by theupward polarization of the ferroelectric layer and the low resistance ofthe anisotrophic conduction layer, and a fourth state may be expressedby the upward polarization of the ferroelectric layer and the highresistance of the anisotrophic conduction layer.

According to another aspect of the present invention, there is provideda writing method for a ferroelectric recording medium, the writingmethod including: applying one of a first voltage, a coercive voltageand a second voltage to a ferroelectric recording medium to change apolarization state of a ferroelectric layer and a resistance of theanisotrophic conduction layer of the ferroelectric recording medium,thereby writing multi-bit information on the ferroelectric recordingmedium.

A first state may be expressed by applying the coercive voltage to theferroelectric recording medium such that the ferroelectric layer has adownward polarization and the anisotrophic conduction layer has a lowresistance.

Before expressing the first state, the first voltage may be applied tothe ferroelectric recording medium such that the resistance of theanisotrophic conduction layer decreases.

A second state may be expressed by applying the second voltage to theferroelectric recording medium such that the ferroelectric layer has adownward polarization and the anisotrophic conduction layer has a highresistance.

A third state may be expressed by applying a negative voltage, amagnitude of which is equal to that of the coercive voltage, to theferroelectric recording medium such that the ferroelectric layer has anupward polarization and the anisotrophic conduction layer has a lowresistance.

Before expressing the third state, a negative voltage of which magnitudeis equal to that of the first voltage may be applied to theferroelectric recording medium such that the resistance of theanisotrophic conduction layer decreases.

A fourth state may be expressed by applying a negative voltage, amagnitude of which is equal to that of the second voltage, to theferroelectric recording medium such that the ferroelectric layer has anupward polarization and the anisotrophic conduction layer has a highresistance.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects of the present invention will become moreapparent by describing in detail exemplary embodiments thereof withreference to the attached drawings in which:

FIG. 1 is a perspective view of a related art ferroelectric recordingmedium;

FIG. 2 is a perspective view of another related art ferroelectricrecording medium;

FIG. 3 is a view illustrating a reading/writing method for a related artferroelectric recording medium;

FIG. 4 is a perspective view of a ferroelectric recording mediumaccording to an exemplary embodiment of the present invention;

FIG. 5 is a graph illustrating transition of a nonvolatile anisotrophicconduction layer in the ferroelectric recording medium according to anexemplary embodiment of the present invention;

FIG. 6 is a view illustrating a writing method for the ferroelectricrecording medium according to an exemplary embodiment of the presentinvention; and

FIG. 7 is a view illustrating a reading method for the ferroelectricrecording medium according to an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

The present invention will now be described more fully with reference tothe accompanying drawings, in which exemplary embodiments of theinvention are shown.

Although an applied voltage is illustrated in a step form, this ismerely an example and the voltage may, for example, have a continuousform.

Also, for the convenience of explanation, each domain of a ferroelectriclayer and each of an anisotrophic conduction layer are separatelypartitioned. It is noted that each state of the domains can beimplemented on one domain.

FIG. 4 is a perspective view of a ferroelectric recording mediumaccording to an exemplary embodiment of the present invention.

Referring to FIG. 4, the ferroelectric recording medium 100 includes anelectrode 101, a ferroelectric recording layer 110 formed on theelectrode 101, and an anisotrophic conduction layer 120 covering theferroelectric recording medium 110. Reference numerals 111, 112, 113 and114 represent domains of the ferroelectric recording layer 110, andreference numerals 121, 122, 123 and 124 represent domains of theanisotrophic conduction layer 120, which correspond to the domains 111,112, 113 and 114.

The ferroelectric recording layer 110 performs a read/write operation inassociation with a read/write head 102. Each of the domains 111, 112,113 and 114 can store information independently in association with theread/write head 102. Different data can be recorded on the ferroelectricrecording layer 110 depending on an applied voltage and itspolarization. This feature will be described in detail later.

The anisotrophic conduction layer 120 covers the ferroelectric recordinglayer 110 such that the read/write head 102 does not directly come incontact with the ferroelectric recording layer 110. Since theanisotrophic conduction layer 120 protects the ferroelectric recordinglayer 110 from being scratched, damage of data can be prevented. Also,since the anisotrophic conduction layer 120 protects the read/write headfrom being damaged or broken, degradation of read/write performance canbe prevented.

Since the anisotrophic conduction layer 120 becomes conductive at morethan a threshold voltage, it can serve as an electrode. Therefore, evenwhen a low voltage is applied to the read/write head 102 and theferroelectric recording layer 110, spontaneous polarization of theferroelectric recording layer 110 can be reversed. Consequently,information can be recorded on the ferroelectric recording layer 110even when a low voltage is applied thereto.

Also, the anisotrophic conduction layer 120 can store information inassociation with the ferroelectric recording layer 110. This will bedescribed later.

FIG. 5 is a graph illustrating a transition of the nonvolatileanisotrophic conduction layer 120 in the ferroelectric recording medium100 according to an exemplary embodiment of the present invention.

Referring to FIG. 5, the anisotrophic conduction layer 120 of theferroelectric recording medium 100 has a nonvolatile property and itselectric resistance varies with a voltage applied between the read/writehead 102 and the electrode 101. For the sake of convenience, Vth1 andVth2 are defined as a first voltage and a second voltage, respectively.

The anisotrophic conduction layer 120 is in a nonconductive state untilthe applied voltage reaches the first voltage of Vth1. When the appliedvoltage is greater than or equal to the first voltage of Vth1, theelectric resistance of the anisotrophic conduction layer 120 decreasesso that the anisotrophic conduction layer 120 becomes conductive. Inthis state, the anisotrophic conduction layer 120 maintains itsconductivity even when the applied voltage is removed. Meanwhile, whenthe applied voltage is greater than or equal to the second voltage ofVth2, the electric resistance of the anisotrophic conduction layer 120increases so that the anisotrophic conduction layer 120 becomesnonconductive.

FIG. 6 is a view illustrating a writing method for the ferroelectricrecording medium according to an exemplary embodiment of the presentinvention.

Referring to FIG. 6, a polarization of the ferroelectric recording layer110 and/or a resistance of the anisotrophic conduction layer 120 arechanged by applying one of the first voltage, a coercive voltage and thesecond voltage to the ferroelectric recording medium 100. In this case,one domain can express four states and store two bits.

According to the present invention, four states can be expressed on onedomain, but this is merely an example for an exemplary embodiment of theinvention. That is, the number of expressible states can increase byfurther providing another ferroelectric layer, another anisotrophicconduction layer, and/or separate layers on one domain. Therefore, thestates corresponding to bits can be expressed by forming theferroelectric layer, the anisotrophic conduction layer, and/or theseparate layer in a predetermined layered structure.

Vc represents a voltage when the polarizations of the domains 111, 112,113 and 114 of the ferroelectric layer 110 are reversed by theexternally applied voltage. The voltage Vc is defined as the coercivevoltage. Vth1 is a voltage when the electric resistance of the domains121, 122, 123 and 124 of the anisotrophic conduction layer 120decreases. The voltage Vth1 is defined as the first voltage. Vth2 is avoltage when the electric resistance of the domains 121, 122, 123 and124 increases. The voltage Vth2 is defined as the second voltage. Also,Vs is a surface potential of each layer.

In this exemplary embodiment, a relationship among these voltages isVs<Vth1<Vc<Vth2.

It may be preferable to form a plurality of domains 111, 112, 113 and114 of the ferroelectric layer 110 and a plurality of domains 121, 122,123 and 124 of the anisotrophic conduction layer 120. Also, may bepreferable that the domains 111, 112, 113 and 114 of the ferroelectriclayer 110 and the domains 121, 122, 123 and 124 of the anisotrophicconduction layer 120 are arranged corresponding to one another.

As illustrated in FIG. 6, the domain 111 of the ferroelectric layer 110and the domain 121 of the anisotrophic conduction layer 120 express afirst state. The first state will now be described in detail.

An external positive voltage that is greater than or equal to thecoercive voltage or less than the second voltage is applied to thedomain 111 of the ferroelectric layer 110 and the domain 121 of theanisotrophic conduction layer 120. In this case, the domain 111 of theferroelectric layer 110 has a downward polarization and the domain 121of the anisotrophic conduction layer 120 has a low resistance, so thatthe domain 121 of the anisotrophic conduction layer 120 becomesconductive. In this state, since the electric resistance of the domain121 of the anisotrophic conduction layer 120 is very slight, the statesof the domains 121 and 111 are determined according to the downwardpolarization that exists in the domain 111 of the ferroelectric layer110.

The state of the domains 111 and 121 is defined as the first state. Thedomains 111 and 121 of the first state can configure two bits. Forexample, “0” can be configured in the domain 111 of the ferroelectriclayer 110 and in the domain 121 of the anisotrophic conduction layer120, respectively.

Before expressing the first state, an external voltage that is greaterthan or equal to the first voltage or less than the coercive voltage isapplied to the domain 111 of the ferroelectric layer 110 and the domain121 of the anisotrophic conduction layer 120. In this case, the electricresistance of the domain 121 of the anisotrophic conduction layer 120decreases so that information is easy to store in the domain 111 of theferroelectric layer 110.

As illustrated in FIG. 6, the domain 112 of the ferroelectric layer 110and the domain 122 of the anisotrophic conduction layer 120 express asecond state. The second state will now be described in detail.

An external voltage that is equal to the second voltage or greater thanthe coercive voltage and the second voltage is applied to the domain 112of the ferroelectric layer 110 and the domain 122 of the anisotrophicconduction layer 120. In this case, the domain 112 of the ferroelectriclayer 110 has a downward polarization and the domain 122 of theanisotrophic conduction layer 120 has a high resistance, so that thedomain 122 of the anisotrophic conduction layer 120 becomesnonconductive. In this state, since the electric resistance of thedomain 122 of the anisotrophic conduction layer 120 is very large, thestates of the domains 122 and 112 are determined according to theelectric resistance and the downward polarization of the domain 112 ofthe ferroelectric layer 110.

The state of the domains 112 and 122 is defined as the second state. Thedomains 112 and 122 of the second state can configure two bits. Forexample, “0” can be configured in the domain 112 of the ferroelectriclayer 110 and “1” can be configured in the domain 122 of theanisotrophic conduction layer 120, respectively.

As illustrated in FIG. 6, the domain 113 of the ferroelectric layer 110and the domain 123 of the anisotrophic conduction layer 120 express athird state. The third state will now be described in detail.

An external negative voltage of which magnitude is greater than or equalto the coercive voltage or less than the second voltage is applied tothe domain 113 of the ferroelectric layer 110 and the domain 123 of theanisotrophic conduction layer 120. In this case, the domain 113 of theferroelectric layer 110 has an upward polarization and the domain 123 ofthe anisotrophic conduction layer 120 has a low resistance, so that thedomain 123 of the anisotrophic conduction layer 120 becomes conductive.In that state, since the electric resistance of the domain 123 of theanisotrophic conduction layer 120 is very slight, the states of thedomains 123 and 113 are determined according to the upward polarizationthat exists in the domain 113 of the ferroelectric layer 110.

The state of the domains 113 and 123 is defined as the third state. Thedomains 113 and 123 of the third state can configure two bits. Forexample, “1” can be configured in the domain 113 of the ferroelectriclayer 110 and “0” can be configured in the domain 123 of theanisotrophic conduction layer 120, respectively.

Before expressing the third state, an external negative voltage of whichmagnitude is greater than or equal to the first voltage or less than thecoercive voltage is applied to the domain 113 of the ferroelectric layer110 and the domain 123 of the anisotrophic conduction layer 120. In thiscase, since the electric resistance of the domain 123 of theanisotrophic conduction layer 120 decreases so that information is easyto store in the domain 113 of the ferroelectric layer 110.

As illustrated in FIG. 6, the domain 114 of the ferroelectric layer 110and the domain 124 of the anisotrophic conduction layer 120 express afourth state. The fourth state will now be described in detail.

An external negative voltage of which magnitude is equal to the secondvoltage or greater than the coercive voltage and the second voltage isapplied to the domain 114 of the ferroelectric layer 110 and the domain124 of the anisotrophic conduction layer 120. In this case, the domain114 of the ferroelectric layer 110 has an upward polarization and thedomain 124 of the anisotrophic conduction layer 120 has a highresistance, so that the domain 124 of the anisotrophic conduction layer120 becomes nonconductive. In that state, since the electric resistanceof the domain 124 of the anisotrophic conduction layer 120 is large, thestates of the domains 124 and 114 are determined according to theelectric resistance and the upward polarization of the domain 114 of theferroelectric layer 110.

The state of the domains 114 and 124 is defined as the fourth state. Thedomains 114 and 124 of the fourth state can configure two bits. Forexample, “1” can be configured in the domain 114 of the ferroelectriclayer 110 and the domain 124 of the anisotrophic conduction layer 120,respectively.

As described above, four states can be expressed on one domain pair ofthe ferroelectric layer 110 and the anisotrophic conduction layer 120.Two bits can be configured on one domain pair.

FIG. 7 is a view illustrating a reading method of the ferroelectricrecording medium according to an exemplary embodiment of the presentinvention.

Referring to FIG. 7, different information is stored in the domains 111,112, 113 and 114 of the ferroelectric layer 110 and the domains 121,122, 123 and 124 of the anisotrophic conduction layer 120 according tothe magnitude of the applied external voltage. Corresponding informationis read through the read/write head 102 according to the polarization ofthe domains 111, 112, 113 and 114 and the electric resistance of thedomains 121, 122, 123 and 124.

Voltages V1, V2, V3 and V4 are applied to the read/write head 102 in thedomains 111, 112, 113 and 114 and the corresponding domains 121, 122,123 and 124. Induced currents I1, I2, I3 and I4, which correspond to thevoltages V1, V2, V3 and V3, respectively, are generated in theread/write head 102.

In FIG. 7, the first state is expressed on the domain 111 of theferroelectric layer 110 and the domain 121 of the anisotrophicconduction layer 120. The second state is expressed on the domain 112 ofthe ferroelectric layer 110 and the domain 122 of the anisotrophicconduction layer 120. The third state is expressed on the domain 113 ofthe ferroelectric layer 110 and the domain 123 of the anisotrophicconduction layer 120. Likewise, the fourth state is expressed on thedomain 114 of the ferroelectric layer 110 and the domain 124 of theanisotrophic conduction layer 120.

The first state through the fourth state apply the voltages V1, V2, V3and V4 to the read/write head 102 according to the upward or downwardpolarization existing on the domains of the ferroelectric layer 110 andthe electric resistance of the domains of the anisotrophic conductionlayer 120. In that case, the induced currents I1, I2, I3 and I4corresponding to the voltages V1, V2, V3 and V4 are generated from theread/write head 102.

According to the exemplary embodiment, the induced currents can havefour values and the information corresponding to the respective inducedcurrents can be read.

In the writing operation, information corresponding to the inducedcurrent I1 is stored in the domain pair 111 and 121 when the externalpositive voltage that is greater than or equal to the coercive voltageor less than the second voltage is applied to the domain 111 of theferroelectric layer 110 and the domain 121 of the anisotrophicconductive layer 120. The information may be, for example, 00.

Information corresponding to the induced current I2 is stored in thedomain pair 112 and 122 when the external positive voltage that is equalto the second voltage or greater than the coercive voltage and thesecond voltage is applied to the domain 112 of the ferroelectric layer110 and the domain 122 of the anisotrophic conductive layer 120. Theinformation may be, for example, 01.

Information corresponding to the induced current I3 is stored in thedomain pair 113 and 123 when the external negative voltage of whichmagnitude is greater than or equal to the coercive voltage or less thanthe second voltage is applied to the domain 113 of the ferroelectriclayer 110 and the domain 123 of the anisotrophic conductive layer 120.The information may be, for example, 10.

Information corresponding to the induced current I4 is stored in thedomain pair 114 and 124 when the external negative voltage of whichmagnitude is equal to the second voltage or greater than the coercivevoltage and the second voltage is applied to the domain 114 of theferroelectric layer 110 and the domain 124 of the anisotrophicconductive layer 120. The information may be, for example, 11.

As described above, four states can be expressed on one domain pair ofthe ferroelectric layer 110 and the anisotrophic conduction layer 120,and two bits configured on one domain pair can be reproduced.

According to the present invention, four or more states can be expressedon one domain pair of the ferroelectric layer and the correspondinganisotrophic conduction layer, and two or more bits can be configured onone domain pair.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

1. A ferroelectric recording medium comprising: a ferroelectric layerwhich reverses its polarization when receiving a coercive voltage; and anonvolatile anisotrophic conduction layer having increased electricalconductivity in a vertical direction and disposed on the ferroelectriclayer, wherein the electrical resistance of the anisotrophic conductionlayer decreases when receiving a first voltage lower than the coercivevoltage, and the electrical resistance of the anisotrophic conductionlayer increases when receiving a second voltage higher than the coercivevoltage, and wherein multi-bit information is stored by a combination ofupward and downward polarizations of the ferroelectric layer and highand low electrical resistances of the anisotrophic conduction layer. 2.The ferroelectric recording medium of claim 1, wherein a first state isexpressed by the downward polarization of the ferroelectric layer andthe low resistance of the anisotrophic conduction layer; a second stateis expressed by the downward polarization of the ferroelectric layer andthe high resistance of the anisotrophic conduction layer; a third stateis expressed by the upward polarization of the ferroelectric layer andthe low resistance of the anisotrophic conduction layer; and a fourthstate is expressed by the upward polarization of the ferroelectric layerand the high resistance of the anisotrophic conduction layer.
 3. Theferroelectric recording medium of claim 1, wherein: the ferroelectriclayer is partitioned into a plurality of domains for recording bitinformation; the nonvolatile anisotrophic conduction layer ispartitioned into a plurality of domains for recording bit information;and each of the domains of the ferroelectric layer corresponds to one ofthe domains of the anisotrophic conduction layer.
 4. A ferroelectricrecording medium comprising: a ferroelectric layer which reverses itspolarization when receiving a coercive voltage; and a nonvolatileanisotrophic conduction layer which is disposed on the ferroelectriclayer, wherein the anisotrophic conduction layer is configured toexhibit higher resistance and a nonconductive state, if a receivedvoltage is lower than a first voltage, wherein the anisotrophicconduction layer is configured to exhibit a lower resistance and aconductive state, if the received voltage is greater than or equal tothe first voltage, but less than a second voltage, wherein theanisotrophic conduction layer is configured to exhibit higher resistanceand a nonconductive state if the received voltage is greater than orequal to the second voltage, wherein the first voltage is less than thesecond voltage, wherein the first voltage is less than the coercivevoltage, wherein the coercive voltage is less than the second voltage,and wherein multi-bit information is stored by a combination of upwardand downward polarizations of the ferroelectric layer and higher andlower electrical resistances of the anisotrophic conduction layer. 5.The ferroelectric recording medium of claim 1, wherein the nonvolatileanisotrophic conduction layer has exhibits increased electricalconductivity in the vertical direction relative to electricalconductivity in a horizontal direction.
 6. The ferroelectric recordingmedium of claim 5, wherein the nonvolatile anisotrophic conduction layerexhibits electrical conductivity in the vertical direction but exhibitsno electrical conductivity in the horizontal direction.